I've just checked an existing application written on version 4.0.7 where scoped CSS seemingly works, but I still see x-body class applied to BODY. For some lucky reason it did not screw up styles in that particular application which made me think that x-body wasn't there.

That makes me think that there was probably never an ExtJS version where x-classes were not applied to BODY and HTML when using CSS scoping .

Ran into the same issue today.
The problem hardly causes problems when within a component layout. But we use the scopeResetCss option to be able to create windows and other components on top of an existing website to display movable administrative tools. With x-body applied in some cases (where we do not explicitly set the attributes in the sites styles) the x-body definition with font-family, font-size and color specified break our original site layout.
I guess a working fix would be to move the styles from x-body to x-reset, since x-reset is created around all components anyway.

As you can see I've tried all the suggestions so far, and the classes are still there. Does anyone have any explanation as to what scopeResetCss is actually for if not for getting rid of the body class?

I agree with the previous post: what does a JS library has to write something in the BODY of the page!?
By default this kind of functionality should be disabled and enabled on request not the other way around.